The effective removal of slag, dross and other potentially harmful inclusions from molten metal during the casting process has conventionally relied on a wide spectrum of molten metal filters that capture the impurities by physical means. For example, reticulated ceramic foam filters utilize a torturous path principle whereby as the molten metal is forced to travel through the random nooks and crannies of the filter, many of the particulate inclusions are trapped within interior cavities and passages. In a similar manner, cellular or honeycomb molten metal filters act as sieves or screens that catch large particle inclusions that are too large to pass through the pore openings of the filters.
While screen-based filtration techniques have been widely used, they are ineffective in capturing small inclusions that pass through the pores of the filters. Furthermore, they are structurally unable to increase the molten metal throughput without a corresponding decrease in filtration efficiency. Smaller size inclusions continue to be a problem for foundries and metal casters despite the widespread use of sieving filters. Such inclusions can be detrimental in various castings, particularly castings used in aerospace and other demanding applications. Similarly, while most metal casting producers may desire increases in molten metal throughput per production run, very few are willing to accept the higher potential scrap rate that could occur in switching to larger pore size molten metal filters which would in turn allow additional inclusions to pass through.
Phenolic-resin treated silica mesh filter cloths have also been used to remove inclusions from cast iron. As the molten iron comes in contact with the resin-treated silica cloth, pyrolysis of the resin takes place, creating Fe2SiO4 that coats the fabric and increases the ability of the filter cloth to capture inclusions. The iron silicate provides a sticky surface that captures and holds slag particulates that are small enough to pass through the mesh holes of the material, and thereby increases the overall efficiency of the filter. Iron silicate also has the ability to form solid solutions with some of the specific impurities unique to different types of cast iron. However, the formation of the iron silicate occurs during the molten ferrous alloy casting process and requires the use of a silica-containing filter in combination with the ferrous alloy. It would be desirable to improve filtration capability by providing controlled amounts of active coatings on various different types of molten metal filters such as reticulated ceramic foam, cellular/honeycomb and the like.